The present invention relates to an electrostatic type ink-jet printer and more particularly a device for correcting ink dot misplacement by correctly synchronizing the timing for charging an ink drop emitted through a nozzle of an ink drop generator in response to a signal representative of the print starting point on a recording medium.
In an electrostatic ink-jet printer of the type in which a rotary drum is rotated about its axis and an ink drop print head is transported in parallel with the axis of the rotary drum, printing is started in response to a signal which is generated each revolution of the rotary drum. However, in practice it is extremely difficult to attain exact synchronization of the rotation of the rotary drum with an ink dot printing frequency. In addition, due to the variations in rotation of the rotary drum, it frequently occurs that when the printing is started, an ink dot is placed at a position spaced apart from an intended position by one dot at the most. In order to overcome this drawback, there has been devised a system in which the ink dot placement frequency is increased above a normal running frequency so that the interval between dots is reduced and subsequently ink dot misplacement is reduced. However, the increase in ink dot placement frequency is limited, so that the printing speed is objectionally reduced.
With the ink-jet printer of the type described above, misplacement of ink drops is caused by aerodynamic and electrostatic disturbances while the drops are in flight. For instance, in case of two consecutive ink drops, the succeeding ink drop feels aerodynamic and electrostatic disturbances of the preceding ink drop, so that the succeeding drop is not placed at an intended position. In order to overcome this drawback, there has been proposed a system in which so-called "guard drops" which are not charged are left between pairs of ink drops that are charged and deflected, but this system is not satisfactory in practice.